Hmmmm,
We are chasing each other today! Hungarian Riffles, a fairly common type of riffle, allow the water to go over the top of them but on the back side the water looses pressure or force due to the hollow back side (if you will) of the sluice. It is at this point where the water stalls for a second or two that the heavy materials fall out of the now not so rushing water. Yes, drop riffles allow the gold to just fall out as again there is a momentary pressure change over the riffle. Yes again as both types of riffles can be used in the design of a sluice. The metal Keene sluices use two types of riffles both of which stand above the bottom of the sluice, they do not use drop riffles except possibly in their plastic sluice.
If you've taken a look at Astrobouncer's drop riffle designs especially the latest ones your will see he's really been considering a variety of options in riffle configuration and he must Test these latest ideas of his out to determine what works best. He has been very happy with just simple saw kerfs in the top of a board though he has intermixed narrow kerfs with wide ones and then back again to narrow and up again to wide, I think this traps small and larger pieces of gold. Do you have access to sheetmetal forming equipment or are you knowledgeable in sheetmetal forming, if so you should be able to do wonders with sluice designs in lightweight aluminum.
Keep that brain of yours churning as you are coming up with good questions, 63bkpkr
